Participation in this study will require extensive pre-surgical screening to determine eligibility for DBS surgery, a DBS surgical procedure, and regular follow-ups. Subjects will be seen monthly post surgery for 6 months. After 6 months, data will be assessed and RBS may be offered as a stimulation setting. If so, the stimulator settings will be changed from chronic to responsive. If not, the subject will continue to receive chronic DBS stimulation. Subsequent visits will be scheduled every 6 months until a total of 24 months of study participation.